Gas discharge plasma tube having a multiturn primary winding

ABSTRACT

A PLASMA TUBE CONTAINING A GAS WHICH IS EXCITED BY MEANS OF A MULTITURN COAXIAL PRIMARY WINDING IS DESCRIBED. THE TUBE IS TOPOLOGICALLY TOROIDAL IN SHAPE AND IS FILLED WITH THE GAS. THE TUBE IS SURROUNDED BY A SERIES OF CONDUCTIVE LAYERS, FOR EXAMPLE, LAYERS OF COPPER, WHICH ARE SEPARATED FROM EACH OTHER BY INSULATION. THE CONDUCTIVE LAYERS ARE CONNECTED TOGETHER TO FORM A COAXIAL SERIES MULTITURN WINDING WHICH IS USED AS A PRIMARY WINDING TO EXCITE THE GAS IN THE TUBE.

ited States .atent Inventor Charles B. Zarowin Bronx, Appl No. 814,088 Filed Apr. 7, 1969 Patented June 28, I971 Assignee International Business Machines Corporation Armonk, N.Y.

GAS DISCHARGE PLASMA TUBE HAVING A MULTITURN PRIMARY WINDING 5 Claims, 5 Drawing Figs.

U.S.CI 315/57, 3l3/l53,3l3/l6l, 313/231, 315/1 I 1, 315/236, 3l5/248,331/94.5

Int. Cl IIOlj 7/44, HOlj 65/00, 11015 3/09 Fieldol'Search 313/153,

References Cited UNITED STATES PATENTS Waniek Blackman Kolb Ducati.

Kolb

Primary Examiner-John Kominski Assistant ExaminerC. R. Campbell Attorneys-Hanifin and Jancin and John J. Goodwin ABSTRACT: A plasma tube containing a gas which is excited by means of a multiturn coaxial primary winding is described. The tube is topologically toroidal in shape and is filled with the gas. The tube is surrounded by a series of conductive layers, for example, layers of copper, which are separated from each other by insulation. The conductive layers are connected together to form a coaxial series multitum winding which is used as a primary winding to excite the gas in the tube.

PATENTED JUN28 I97| SHEET 1 OF 2 ATTORNEY GAS DISCHARGE PLASMA TUBE HAVING A MULTITURN PRIMARY WINDING BACKGROL ND OF I HE INVENTION 1. Field of the Invention The present invention is in the field of plasma physics and. more particularly relates to devices for exciting gas into the plasma state. The invention also relates to gas lasers in that the plasma may serve as the active medium for a gas laser.

2. Description of the Prior Art A Study ofa High Current Toroidal Ring Discharge by A. A. Ware, Phil. Trans. of the Roy Soc. A243, 197 (1951) describes a hollow glass torus filled with gas, the outside of which is coated with a layer of copper, except for a small gap.

The present invention is distinct from the prior art in that a multitum coaxial primary is employed. An unobvious result is obtained in that the inductance of the multitum primary winding of the present invention is not a function of the turns ratio, as is usual for transformers.

SUMMARY OF THE INVENTION Production of high current discharges are very important for general plasma research, fusion research and short wavelength ionic lasers. These plasmas are usually obtained by discharging high voltage capacitors through the gas. The distributed circuit of such a discharge has an equivalent lumped RLC circuit. The peak current obtained is higher for a damped than an underdamped discharge. A damped discharge occurs when 4L R C, where L is the inductance,R is the resistance and C is the capacitance of the lumped equivalent. The resistance of the plasma is quite low at high current densities l"-'Il) and L is generally difficult to keep below-l00 nh. Thus, the capacitance required for a critically damped discharge (where the equality holds) is C=4L/R 4(10- )/l0* =4 Farads. This is an extremely large capacitance, which at high voltage represents a great deal of stored energy. For this reason, prior use of these discharges involved only underdamped discharges, where the peak currents are substantially smaller and much of the energy is wasted since voltage and current are out ofphase.

A stepdown transformer between the plasma and the capacitor will make the capacitor appear larger to the plasma resistance by the square of the turns ratio n. The inequality for the damped discharge then becomes: nCRZ 4L (1-K where K is the coefficient of coupling, allowing the use of a smaller capacitor by a factor of n /lK while still maintaining a damped discharge. However, most transformers have a leakage inductance, (lK )L,,, which exceeds the inductance of 100 nh. by at least an order of magnitude and thus requires still higher turns ratio to maintain the conditions for damped discharge.

An object of the present invention is to provide a multitum stepdown transformer for a discharge device having a low total inductance which will produce high current discharges. Another object of the present invention is to provide a multiturn stepdown transformer wherein the primary inductance is independent of the turns ratio rather than the square of the turns ratio.

A further object of the present invention is to provide a multiturn coaxial primary coil for a discharge device by surrounding the device with alternate layers of conductive material connected in series and separated by layers of insulating material.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 shows a cross section view of a plasma tube having a multiturn coaxial primary winding in accordance with the principles of the present invention.

FIGS. 2, 3 and 4 are schematic illustrations of transformer circuits useful in explaining the present invention.

FIG 5 is a simplified representation of a cross section of the device of FIG. 1

- DESCRIPTION OF THE PREFERRED EMBODIMENT As stated in the summary, high current discharges are required for exciting the gas in a plasma tube or ionic laser into the plasma state. The high current discharges are obtained by discharging high voltage capacitors through the gas. The dynamics of the discharge of a capacitor into a gas de pends on the properties of the associated RLC circuit where L is the inductance intrinsic to the capacitor, the connecting electrical leads and to the discharge itself. R is mainly the resistance of the plasma and C is mainly the capacitance of the capacitor. The peak discharge current is higher for a damped discharge, and a damped discharge occurs when 4LR C. Thus, for a damped discharge, the capacitor has the value C2 4L/R In an underdamped discharge, the charge oscillates back and forth between the capacitance C and the inductance L, and only a small portion of the charge is dissipated in the gas for each oscillation. It, therefore, takes a relatively long time for the charge to dissipate. A damped discharge is preferred because the charge dissipates rapidly. However, sincethe resistance of the plasma R is relatively low and the v inductance of the circuit L is relatively high, a prohibitively large capacitor would be required for damped discharge. Thus, prior devices involved the use of inefficient underdamped discharges.

A given capacitance can be made to appear larger to the plasma resistance by employing a stepdown transformer between the capacitor and the plasma. For a transformer turns ratio n, the relationship for a damped discharge is This would appear to permit the use of a capacitor which is smaller by a factor of n However, conventional transformers have a leakage inductance of at least 1 microhenry, which, in turn, raises the value of capacitance required. Further increasing the turns ratio does not solve the problem because the leakage inductance will further increase.

Referring to FIG. 1, a schematic view of a section of an nturn stepdown transformer for a gas discharge device which has low total inductance and the property that the primary inductance is independent of the turns ratio rather than the square of the turns ratio. In FIG. 1, the topologically toroidal plasma tube is shown consisting of a straight portion 10 and a curved portion 12. Portions l0 and 12 are circular in cross section and are constructed of glass, ceramic or the like. Straight portion 10 is shown having extended open end sections since the device of FIG. 1 may be employed as an ionic laser and the end sections are provided to permit the laser light to be coupled out of the tube via Brewster windows, etc. The plasma tube contains a suitable gas, such as argon, chlorine, oxygen, etc., and forms a closed loop single turn secondary transformer winding. An n-turn coaxial primary winding is formed around the plasma tube portions 10 and 12. The primary winding is shown in a sectional view taken through the center of the winding in the plane of the paper.

The winding is formed by a series of layers of conductive material, such as copper, separated by insulation and connected in series circuit. A first conductive input connector 14 is connected to the first layer (or winding) 16. Winding l6 surrounds the straight tube portion 10 from left to right in FIG. 1 and extends back along the curved portion 12 from right to left and terminates just above the straight portion 10 without closing on itself. A second winding layer 18 is provided which is identical to winding 16 except it is larger in diameter and does not connect to input connector 14. Winding layer 18 is separated from layer 16 by insulating material which is not crosshatched in FIG. 1 for clarity. At one location, the insulation is omitted and winding layer 16 is connected in series to winding layer 18 by conductive connector 20.

Additional winding layers are constructed in a similar manner For example. a third winding layer 22 is shown separated by insulation from Indlllg layer 18 except at connection 24 Ifa turns ratio of three lS desired. winding layer'22 is made the final layer and is connected to the second conductive input connector 26. If a higher turns ratio is desired, further layers of insulation and conductive material may be. added in the manner described. The device of FIG. 1 is a series connected multiturn coaxial primary transformer winding connected to a single turn secondary winding in the form ofa gas filled closed loop plasma tube. The input connectors 14, and 26 are connected to a suitable capacitor 28 which discharges and provides the input current to the multiturn coaxial primary winding.

The properties of the subject invention can be described more specifically in terms of usual transformers. FIG. 2 shows a schematic ofa transformer having an n-turn primary winding 28, a single turn secondary winding 30 and a load resistance R. The primary and secondary windings have a coupling coefiicient K. For K not too different from one, the equivalent of the circuit of FIG. 2 is shown in FIG. 3 with all components referred to the primary. The primary inductance L, is defined by L,,=,,li where 1 is the flux produced by the primary circuit i and the leakage inductancer(l-l( )L,, is expressed in terms of the coupling coefficient K. The secondary inductance L, is related to L by L,=L,,/n At high frequencies, or for short duration pulses, theprimary inductance L appears as an open circuit if( ll( is much less than (one), and n R wL thus the resulting RLC circuit, as shown in FIG. 4, has the advantage that the requirement for damped discharge that 4(1- l( )L,, be less thanor equal to (n R) C is satisfied for a given R, L,,, C and K by selecting an appropriate value of n.

As shown in FIG. 1, the leakage inductance of a conventional transformer is equal (l--ll(")L,,. However, L, is equal to "di /1,, where 1),, is the total primary flux. In the transformer. the flux 1 for each turn adds, so the D, is equal to nCDand the leakage inductance is a function of the turns ratio. Also, the secondary current I, is equal to NI In the device of the present invention, because of the multiturn coaxial configuration, the current in the first or innermost conductor is I,-.l,/n, in the next conductor the current is I,2I,/n, and so on, to the n" conductor where l,-nl,/n=0. Because of this, it can be shown that the leakage inductance for the device of the present invention is expressed as where r is the radius of the toroid 10 cross section, Ar is the distance between the coaxial layers 16, 18 and 22, l is the length of the coaxial discharge, that is, the average length of the toroid, and where p is the magnetic permeability of the vacuum. The ratio Ar/r, is small, therefore is much less than one and the value of leakage inductance is W- (b) Also for O 1, AT

(c) and so on to a 2M 353i However,

What has been described is an improved gas discharge tube having a multiturn coaxial primary winding which permits the use of a damped discharge without requiring a large capacitor. The device has the unique feature that the leakage inductance is essentially independent of the turns ratio. The device may be employed as an element of an ionic laser.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

I claim:

1, A gas discharge device for exciting a comprising:

a topologically toroidal closed loop tube containing a gas;

a plurality of different diameter sheaths of electrically conductive material arranged coaxially on said closed loop tube;

insulating material located between each of said coaxial sheaths of conductive material to electrically isolate said sheaths of material from each other; and

a plurality of electrically conductive connectors, each connector connecting a separate one of said sheaths of material to the next proximate sheath of material to connect said plurality of sheaths of material into a series electrical circuit.

2. A gas discharge device according to claim 1 further including means connected to said innermost sheath of material and to said outermost sheath of material for applying electrical energy to said plurality of sheaths of material, said electrical energy applied to said plurality of sheaths being coupled to said gas by transformer action.

3 A gas discharge device according to claim 1 wherein said gas into a plasma topologically toroidal closed loop tube includes a linear portion having a circular cross section and a curved portion having a circular cross section, and wherein said plurality ofcoaxial sheaths of material having linear portions having circular cross sections enclosing said linear portion of said tube and curved portions having circular cross sections enclosing said curved portion of said tube. I

4. A transformer for coupling electrical energy from a primary circuit to a secondary circuit comprising:

a secondary circuit including a topologically toroidal tube containing a gas; and a primary circuit including a plurality of coaxial layers of 

